*A traveling wave transverse rf deflecting structure is discussed as a tool for bunch length measurement and as an aid to detailed phase space diagnostics in future FEL projects. The high frequency time variation of the deflecting fields is used to 'pitch' or 'yaw' the electron bunch where the resulting transverse beam width measured on a simple profile monitor represents a reliable, single-shot measure of the absolute bunch length. A small rf phase shift, from the zero-crossing, adds a net centroid kick to the bunch so that it intercepts an off-axis screen. The deflecting rf is then pulsed at a much lower rate than the machine rate, and a pulse-stealing process is used to monitor the bunch length continuously. Multiple screens at appropriate phase advance locations might also be used to monitor the 'slice' (i.e., time-correlated) transverse emittance. A screen placed at a point of significant momentum dispersion can also be used to reveal the time-dependent energy spread. Suitable S-band structures were built and tested in the 1960's at SLAC and one has now been installed in the SLAC linac and configured to measure bunch lengths of test beams. Simulations of these schemes are presented for the Linac Coherent Light Source [1] and the existing SLAC linac.

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The Linac Coherent Light Source (LCLS) is a SASE free electron laser using the last 1/3 of the SLAC two mile linac to produce 1.5 to 15 angstrom x-rays in a 100 meter long undulator for use in a variety of x-ray science experiments. In order to satisfy the demanding electron beam requirements, a new 135 MeV photo-injector will be built in an existing, off-axis vault at the 2/3 point of the main linac. The injector accelerator consists of a BNL/SLAC/UCLA 1.6 cell S-band gun followed by two 3meter long SLAC accelerator sections. The 6MeV beam from the gun is matched into the first accelerator section and accelerated to 135 MeV before injection onto the main linac axis with a 35 degree bend. Several modifications to the rf gun, linac and beamline as well as the inclusion of several diagnostics have been incorporated into the injector design to achieve the required 1.2 micron projected emittance at a charge of 1 nC. In addition, an inverse free electron laser, the laser heater, will be used to increase the uncorrelated energy spread to suppress coherent synchrotron radiation and longitudinal space charge instabilities in the main accelerator and bunch compressors. The configuration and function of the major injector components will be described.